I have a 502 MPI with an MSD 6m box. My engine rpm drops very suddenly then bounces back up...then up and down. If I'm cruising at 3500, it will drop to around 2500-3000 and then go right back up to 3500. There is no pattern. It will run fine for awhile, then act up, then go back to running fine again. I have checked all of my connections. My next step is to hook up my Rinda tool and check for any codes. With a very sudden fluctuation like this, does it sound more like an ignition problem or a fuel problem. Because it's such a suddent sharp drop ,I'm thinking it's ignition, which means I probably won't get any codes...maybe a bad MSD box ????

I'd say ignition. If it was fuel you would think it would backfire as it lost fuel pressure. I've had similar problems with an MSD box on the street.

This shouldn't be hard to diagnose if its ignition,on a mpi motor you can simply unplug the msd box and plug the stock connections for the ignition back together without no real effort and you'll know immediately,Smitty

Exactly!!!!!

You shouldn't need an MSD on that engine, and I doubt its doing anything to change performance.

I'm not saying he necesarily needs or does not need the msd (but your most likely correct) BUT on the mpi motors all the stock ign remains intact when running a msd box so it can be switched back to at least test it by simply switching the connections back to stock. I got QUITE familar with this routine last summer when trying to run a msd marine box on my boat!!

Thanks guys- I'll give that a try